This question piqued my interest so I went back to my sources and did a bit of googling…
My original sources include Osprey MAA 201, 'The British Army on Campaign 4 1882-1902'. This says that most of the troops wore their home service undress with a "white foreign service helmet with puggarees, which were mostly stained brown". It goes on to say, "Of 3/KRRC's black serge, Lt Marling noted, 'We do look a lot of dirty ruffians'." I recall reading at least one other reference to a black uniform and seeing at least one illustration of such a uniform with red facings; however, I can't find these other references now.
A bit of a search today uncovered the below references to 3/KRRC wearing rifle green uniforms in 1882:
BritishEmpire.co.uk has a really useful photograph of a group of 3/KRRC other ranks "at the start of the campaign in 1882" and says, "The uniforms are rifle green serge frocks and trousers." It also shows undyed helmets: link
BritishBattles.com says, "the KRRC fought in rifle green tunics and trousers.": link
The above 2 references, coupled with near-contemporary description and illustrations of KRRC uniform in home service, indicates that perhaps a (very?) dark, rifle green uniform is more accurate and that descriptions of the uniform as 'black' may be because it looked black under various light conditions. Rifle green certainly looks better for scale model soldiers!